Optimization and mechanism of phosphorus removal in plant-biofilm oxidation ditches: Plant uptake, iron plaque adsorption, and rhizosphere regulation

Poor phosphorus removal capacity has always been a Chinese wall to the development of oxidation ditch. Here, plant-biofilm oxidation ditch (PBOD) was proposed with different aeration conditions and plant types to investigate the parameter optimization of PBOD and internal mechanism of accelerated phosphorus removal. The results showed that the combination of aeration and plants was demonstrated as effective to enhance the PBOD capacity for sewage purification and phosphorus removal, since they can jointly promote the change of Fe2+ into Fe3+. Subsequently, part of Fe3+ and phosphate ions formed phosphate precipitation, and the other part of Fe3+ formed iron membrane on root surface, which can effectively adsorb phosphorus in sewage, so as to reduce the content of iron and phosphorus in sewage. Plant species did not play an important role in the removal effect of phosphorus but in the removal pathway: Canna generalis absorbed more phosphorus in the plant tissue than Zizania caduciflora, whereas Zizania caduciflora adsorbed more phosphorus on the root surface than Canna generalis. This is due to (a) Canna generalis roots released more organic acids, which may promote the conversion of insoluble inorganic phosphorus into soluble phosphate ions for plant uptake, and may reduce the amount of iron plaque on the root surface by generating organic acid chelated iron; (b) Zizania caduciflora has strong root oxygen secretion capacity and is easier to form iron plaque on the root surface, so it attaches more phosphorus.